The temperatures you refer to are celcius. The 750C is about
1383F. There is a risk of cracking due to thermal shock. The 300C
is about 572F and hotter than you home oven can reach, or at least
hotter than mine. This is what I do:
Anneal your sterling to dead soft, either but furnace or torch, by
heating to 1200F-1300F and quenching immediately.
Place in a preheated furnace at 600F for 30-45 minutes then remove
and allow to cool on the benchtop.
This has always given me excellent results making money clips.
Hope this helps,